You need to use good quality discs for XGD3 backups. It didn't really matter as much before with XGD2, you could get away with a lot. But not so with XGD3. The reason you need to have good media is because you're now burning to the outer edge of the disc, the most inconsistent area. On lower quality discs, this area is extremely spotty and I'm willing to bet money that's why you're having issues. You need to get your hands on some Made in Singapore Verbs and use those for your XGD3 backups.